The present invention relates to an optical sensing equipment utilizing polarized light, and more specifically to an optical sensing equipment for optically measuring the quantity of light transmitted through an optical fiber.
FIG. 1 is a block diagram illustrating a conventional optical sensing equipment utilizing polarized light. In FIG. 1, there is shown an arrangement of a light source 1, an optical fiber 2, a micro lens 3, and a polarizer 4, wherein the light emitted from the light source 1 is changed by the micro lens 3 into a parallel beam, which passes through the optical fiber 2 before being converted by the polarizer 4 into linearly polarized light.
The arrangement includes a photoelastic element 5, a quarter-wave plate 6, and analyzer 7 for dividing the light into two vertically polarized components, micro lenses 8 and 9, optical fibers 10 and 11, photodetectors 12 and 13, an adder 14, a subtractor 15, and a divider 16.
The photoelastic element 5 is arranged so that a pressure being measured is applied to one face thereof. A double-refractive phenomenon is caused due to the applied pressure. For instance, provided that the photoelastic element 5 is an isotropic medium, the refractive index in the direction in which the pressure is applied becomes different from the refractive indexes in the two directions perpendicular thereto. Accordingly, if a light beam having a field component in the pressure-applied direction and another light beam having a field component perpendicular to the former are simultaneously incident on the photoelastic element 5, a phase difference will occur at the output thereof. For instance, if the linearly polarized light of the output of the polarizer 4 is incident on the photoelastic element 5 at an angle of 45.degree. with respect to the optical axis, the output of the photoelastic element 5 will be elliptically polarized in an amount depending on the pressure.
The elliptically polarized light is imparted by the quarter-wave plate 6 a phase difference of 90.degree. and an optical bias, and then divided by the analyzer 7 into two polarized components perpendicular to each other. The two components are focused by the micro lenses 8 and 9, passed through the optical fibers 10 and 11, and then subjected to photoelectric conversion by the photodetectors 12 and 13.
The sum of and difference between the photodetector outputs are determined by the adder 14 and the subtractor 15, whereas the division of them is carried out by the divider 16. An output is thereby produced indicative of the pressure, which output is not affected by variations in the intensity of the light source 1.
Although the photoelastic element 5 is employed as a sensor element in the aforementioned example in which a pressure is measured, a Faraday element or Pockels element may be used in the case of measuring a magnetic or electric magnetic field.
Since the photomodulated light is transmitted through the two optical fibers 10 and 11 from the optical sensor arrangement composed of the optical fiber 2 through the micro lens 9, the conventional optical sensing equipment thus constructed has such a disadvantage that optical loss fluctuations between the optical fibers 10 and 11 cause errors in measurement.